(1) Field of the Invention
This invention relates to an active carbon formed into a granular or other suitable shape by using bentonite clay as a binder. More particularly, the invention deals with a method for producing a shaped wood-based activated carbon with essentially no pore volume in pores greater than one micron in diameter. Thus, the invention product is particularly well adapted for use as contact mass in adsorption and catalytic processes.
(2) Description of the Prior Art
Granular carbons and carbon pellets are typically used in columns or beds for gas and vapor systems and also for processing a number of liquids. To qualify for this application, a carbon must posses sufficient mechanical strength to withstand the abrasion incident to continued use. Gas-adsorbing carbons should be as dense as is consistent with high adsorptive power so as not to require a large space for the adsorber. The development of high adsorptive power during thermal activation, however, is accompanied by a loss of mechanical strength and density; therefore, some compromise is required in selecting the degree to which the activation is conducted.
Activated carbon currently produced from wood waste has an exceptionally high internal surface area and activity level. However, the granular portion is relatively soft and its shape is irregular. Therefore, application of granular wood-based carbon in general gas phase and liquid phase adsorption is limited by a number of constraints, to wit: (1) its low apparent density results in low volumetric adsorption capacity; (2) its low hardness results in a high fines generation rate (dusting); and (3) its limited maximum particle size and its irregular granular shape both result in a high pressure drop in gas phase applications. It has been found that changes in hardness and shape can be effected by agglomerating or shaping an active wood-based carbon with bentonite clay in the particular manner of the invention which achieves properties which provide shipping advantages by reducing dusting tendancies, as well as properties suited to gas and liquid phase applications. Particularly, invention process provides shaped active carbon the maximum particle size of which is limited only by the equipment used in the shaping process.
Shaping of carbons, generally, is taught in the prior art. U.S. Pat. No. 2,455,509, for example, teaches a method of extruding irregularly shaped carbon rods but does not teach any particular binder material. The organic binders most commonly used are substances which when heat-treated in an inert or non-oxidizing atmosphere yield a high proportion of fixed or residual carbon.
Thus, U.S. Pat. No. 3,454,502 teaches activated carbon tablets formed from powdered activated carbon with water soluble aliphatic petroleum hydrocarbon sulfonate detergent as binder, and U.S. Pat. No. 3,592,779 teaches the use of acid sludge formed by the reaction of a mineral acid with a relatively high molecular weight hydrocarbon as a binder for particulate carbon and the subsequent activation thereof. U.S. Pat. No. 3,864,277 teaches a hard granular activated carbon and preparation by extrusion of a mixture of a carbonaceous material, a binder, and an inorganic activating agent (phosphoric acid). The binders taught are lignosulfonates and polyvinyl alcohols which are soluble or emulsifiable in water or phosphoric acid solutions.
U.S. Pat. No. 3,960,761 discloses a method of production of strong active carbon moldings by extruding or briquetting low ash mineral coals with phenols and aldehydes used as binders. In teaching carbon particulates comprising carbon black spheres and a carbon binder with desirable pore size distribution, U.S. Pat. No. 4,029,600 discloses certain polymers and coal tar pitch as the carbon binder. Also, U.S. Pat. No. 4,051,098 discloses a one step phenol-formaldehyde resin, a modified phenol-formaldehyde resin or a mixture thereof as a binder in a process for manufacturing a shaped active carbon. The patentees note that the resins do not damage the adsorption property of the active carbon due to their markedly high carbonization rates in the high temperature treatment employed in the manufacturing process.
Finally, U.S. Pat. No. 4,124,529 teaches carbonaceous adsorbents produced by shaping the carbon with elastomeric plastics and thermoplastic materials such as polyvinyl alcohol, polypropylene, and polyethylene.
It has now been discovered that bentonite clay, a non-carbon containing material, when mixed with a pulverized, activated wood-based carbon in a liquid and agglomerated or shaped, dried, and heat-treated, performs as an effective carbon binder to produce a shaped active carbon material with higher apparent density and hardness, reduced pressure drop in gas phase applications, and increased volumetric adsorption capacity. Particularly surprising is the increased volumetric adsorption capacity of the shaped product in view of the fact that the weight bases adsorption capacity has been decreased by the presence of the inert clay binder. Advantageously, the extruded carbon product of this invention produces particles of uniform activity throughout. Prior art thermally activated granular carbon particles exhibit high activity on the outer surface with decreasing activity toward the center.